The energy is a lifeline of the development of a country. With the rapid development of social economy, the shortage of commonly-used fossil resources such as petroleum, natural gas, coal, etc. at present is growing. Meanwhile, the use of large amounts of fossil resources results in many hazards to environment, such as green house effect, acid rain, etc. Furthermore, related industries, such as catering industry, food industry, etc., produce lots of waste animal and plant oils every day. Such waste oils generally contain a large amount of impurities, and part thereof is recycled to manufacture soap, part thereof is directly discharged without recycling and treating, thereby causing an environmental pollution, and part thereof goes back to dinner tables. Therefore, it has been paid more and more attention by people to how to sufficiently and effectively utilize waste animal and plant oils, so as to reduce the pollution of waste animal and plant oils to the environment such as water area, etc. and the harm to human body, and guarantee food safety so as to realize resource recycling.
The biodiesel, i.e. fatty acid methyl ester, has chemical formula of RCOOCH3 (wherein R is C12-17), is prepared mainly from plant oils (rice bran oil, cotton seed oil, sea kale oil, peanut oil, rapeseed oil, soybean oil, sunflower seed oil, etc.) or animal fats, is a diesel substitute obtained from renewable biomass resource and having property similar to diesel, and is a kind of renewable biomass resource. The main ingredients of the biodiesel are ester substances, such as methyl ester or ethyl ester formed of long-chain fatty acid. The biodiesel utilizes alcohols substance (methanol or ethanol) to proceed ester exchange reaction (transesterification) with the main ingredient (triglyceride) in natural plant oils or animal fats, substitutes glyceryl on long-chain fatty acid with methoxyl, and breaks triglyceride into multiple long-chain fatty acid methyl esters, so as to shorten the length of carbon chain, reduce the viscosity of oil material, improve the flowability and vaporability of oil material and thereby achieve the requirement of using as fuel.
Compared with fossil diesel, the biodiesel has similar ignition performance, heat value and dynamic property, far less corrosivity to engine and higher safety, thus it can be directly used as fuel without making any modification to existing diesel engine and does not cause any harmful effect on the engine. Moreover, the biodiesel can be mixed with fossil diesel at any ratio, which can not only reduce the emission of substances in tail gas harmful to the environment, but also effectively lower the use cost and reduce the overdependence on the fossil energy.
The biodiesel is derived from renewable biomass resources, thus not containing the substances such as sulfur, aromatic hydrocarbon, etc. commonly found in the fossil diesel. The biodiesel has a short carbon chain, contains oxygen itself and can be completely combusted, so as not to produce atmospheric pollutants such as SOx, aromatic hydrocarbon, polycyclic aromatic hydrocarbon, etc., averagely reduce waste gas by above 45%, and effectively reduce the hazard of tail gas to the environment. The use of biodiesel can effectively reduce the concentrations of CO and TPM in tail gas, and the contribution of the biodiesel to green house effect is only 25% or lower of that of fossil diesel. Moreover, the biodiesel also has excellent biodegradability compared with fossil diesel.
The biodiesel has good environmental protection performance. It has a low sulfur content. The amount of sulfur dioxide and sulfide emitted by the biodiesel upon combustion is reduced by about 30% compared to common diesel. The biodiesel has oxygen content up to 11%, and good ignition performance, so that the harmful substances such as HC, CO, etc. discharged during combustion are greatly reduced compared to common diesel. The substances discharged by the biodiesel upon combustion contain no harmful substances such as sulfur dioxide, lead, halogen, etc, which is very favorable to environmental protection. Hence, the biodiesel is renewable environmentally-friendly fuel, and has good application prospect.
At present, the methods for preparing biodiesel mainly include: 1) esterification method for performing esterification reaction using oils and fats and methanol in the presence of acid catalyst; 2) ester exchange method for performing ester exchange reaction using oils and fats and methanol in the presence of alkali catalyst; 3) as to waste oil having many impurities and especially high free fatty acid contents, such as trench oil, generally esterification is performed first, and then alkali catalyzed ester exchange is performed, instead of directly performing ester exchange reaction by adopting alkali catalyst.
Chinese patent application CN 101906355A discloses a method for preparing biodiesel by utilizing food waste recycling oil. The method comprises pre-treating dining waste recycled oil using 1.5 wt % of 98% concentrated sulfuric acid, then esterifying the pre-treated dining waste recycled oil using 5 wt % of concentrated sulfuric acid based on the weight of raw material under the temperature of 65° C. for 2 h, transferring into an acid-catalyzed settlement separator, performing ester exchange using 1 wt % of 95% solid sodium hydroxide based on the weight of raw material under the temperature of 65° C. for 30 min, separating crude biodiesel through an alkali-catalyzed settlement separator, washing the obtained crude biodiesel with 3% of 98% concentrated sulfuric acid based on the volume of the crude biodiesel in a water-washing neutralization tower, then washing with water, and heating to distill the upper-layer biodiesel to obtain the final product. The method performs esterification reaction and ester exchange reaction in different reaction kettles respectively, and separates intermediate products through two settlement separators, thus increasing equipment cost and energy consumption. The method requires water-washing neutralization after ester exchange. The total amount of added concentrated sulfuric acid reaches 9.5% of raw material, so as to produce a large amount of acid water during water washing process, thus a large amount of alkali is required for neutralization in latter sewage treatment, thereby increasing treatment cost. On the other hand, there exists some problems while independently using sodium hydroxide as catalyst in practical application: 1. long dissolving time and low dissolution rate of sodium hydroxide and methanol; 2. incomplete ester exchange reaction; 3. easy agglomeration of sodium salt, which results in serious pipeline blocking during ester exchange reaction when using soybean acid oil as raw material, thus not widely suitable for various raw materials; and 4. low separation speed of glycerol and fatty acid methyl ester produced in the reaction.
CN 101012388A discloses a method of manufacturing biological diesel oil from trench oil. The method comprises using trench oil and 0.2-3% solid acid catalyst (CaO or SiO2) of porous carrier to perform esterification reaction under high temperature of above 95-130° C. for 1-4 h, then separating the solid acid catalyst, performing ester exchange reaction by using 3-5% solid alkaline catalyst (CaO or MgO) at 50-65° C. for 0.2-2 h, separating biological diesel oil in upper layer, separating the solid alkaline catalyst and glycerol from lower layer, and distilling to recover methanol. The method has a high esterification reaction temperature, and utilizes acid catalyst of porous carrier to increase reaction surface area, so as to accelerate esterification reaction speed. Although the method requires no neutralization and washing, it also requires two separation steps to separate the solid acid catalyst and the solid alkaline catalyst respectively. In addition, the method has no distillation, and no matter how thorough the ester exchange reaction is performed, it is impossible to convert all neutral oils and fats (triglyceride) into fatty acid methyl ester. In the patent, the biological diesel oil product is not subjected to distillation, and inevitably contains monoglyceride, diglyceride and triglyceride which are main ingredients of plant pitch, i.e., the patent does not separate plant pitch and biodiesel.
Therefore, there is a need for a method for preparing biodiesel with low equipment requirement, high conversion rates of esterification reaction and ester exchange reaction, low energy consumption and low cost.